The proliferation of electronic equipment, both for business purposes and for entertainment, has increased the need for small, inexpensive and efficient power supplies. One common type of power supply is a dc-to-dc converter which accepts electrical power at one voltage level, converts it to high frequency alternating current, uses a transformer to both adjust the voltage level and to isolate the power source from the electronic load, and rectifies the resulting alternating current to direct current to operate the electronic equipment. Such converters are simple, low cost means for providing the required power. The reason the intermediate alternating current is at a high frequency is to reduce the size of the required transformer. Such converters are often used to power portable equipment or to recharge batteries in the equipment. Some dc-to-dc power supplies are powered by standard household current rectified to provide the direct current input and taking the form of plug-mounted structures where the entire power supply is supported by the electrical connector pins on the plug.
In power supplies such as plug-mounted converters, size, weight and cost are critical. Reducing the number of components is critical in reducing size and complexity of the converter. It has been found that resonant, self-oscillating dc-to-dc converters may meet many of the requirement of such plug-mounted converter applications. A family of push-pull converters is analyzed in "Nonlinear Analysis of a Family of LC Tuned Inverters" by F. C. Y. Lee and T. G. Wilson, PESC 74 Record, pages 223-231, Jun. 10, 1974. In order to avoid the higher breakdown requirements of the push-pull converter configuration, it has been suggested to utilize a half-bridge zero-voltage-switched configuration to reduce the cross-transistor voltages by half. Such configurations are disclosed in "A New Family of Isolated Zero-Voltage-Switched Converters" by R. Farrington, M. M. Jananovic and F. C. Lee, 1991IEEE 0-7803-0090-4/91/0700-0209, pages 209-215, April 1991, and "Comparative Study of Voltage-Mode Resonant Converters with Transformers" by T. Ninomiya, T. Higashi, M. Nakahara and K. Harada, 1991IEEE 0-7803-0090-4/91-0700-0474, pages 474-480, April 1991.
All of these prior art converters require either expensive and lossy square loop magnetics or complicated control circuitry, thereby increasing the size and weight of converters. Moreover, these prior art converters also have poor short circuit and/or open circuit characteristics, requiring expensive and complicated special overload protection control circuitry.